1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a wireless communication system with femtocells and a method for mitigating inter-cell interference in the wireless communication system.
2. Description of the Related Art
A cellular communication system includes a plurality of macro cells, each defined by a coverage area of a macro base station and providing communication service to mobile stations within the coverage area. Typically, the mobile stations experience different signal environments even within the same macro cell. However, the macro base station serves the mobile stations without taking into account mobile station-specific signal environments, resulting in service quality degradation and delay. In order to mitigate these problems, service providers deploy intermediate devices, such as repeater and remote units for relaying signals between base stations and mobile stations. Because the macro base stations and intermediate devices are the service provider properties, the deployment of intermediate devices increases the business cost and system implementation complexity.
In order to solve these problems, the current communication systems are designed to support femto base stations that can be installed overlappingly within a macro cell. A femto base station is a small cellular base station that can be installed in residential or small business environments as needed. Because the femtocell is installed on the user's premises, it is possible to improve an indoor signal environment and service quality without increasing a fixed infrastructure cost of the service provider.
FIG. 1 illustrates a conventional wireless communication system including femtocells installed within a macrocell.
Referring to FIG. 1, multiple femto base stations 21 are installed within a macro cell of a macro base station 11. Consequently, signals of the macrocell and the femto cells are likely to interfere with each other. Generally, base stations are installed at optimum positions for each specific base station, and it is difficult install the base stations in consideration of the probability of interferences among each other. For example, because each femto base station 21 uses an omnidirectional antenna, the mobile station 31 served by the femto base station 21 is likely to receive signals transmitted by the macro base station 11 and the other femto base station 21, resulting in interference. Also, the signals transmitted by neighbor femto base stations can act as interference to the mobile station 31.